Light-controlled waveguide attenuator



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United States Patent LIGHT-CONTROLLED WAVEGUIDE A'ITEN UATOR BenjaminKazan, Princeton, N. 1., asslgnor to Radio Corporation of America, acorporation of Delaware Application April 20, 1954, Serial No. 424,400

8 Claims. (Cl. 333-81) which may be adjustable in position. Of course, aphysical device does not lend itself to being moved rapidly andfaithfully in response to a controlling force.

ltis an object of this invention to provide a variable attenuator foruse in a waveguide which is capable of varying the attenuation over abroad range at a very rapid rate in response to an electrical controlsignal.

It is another object of this invention to provide a waveguide attenuatorincluding a photoconductive material which changes in conductivity inaccordance with the amount of light falling thereon.

It is a further object of this invention to provide a forked waveguideincluding means to rapidly vary the proportion of the energy directedalong the two forks.

It is a further object to provide an improved means for varying theamount of reflected radio frequency energy from a given point in awaveguide.

It is a still further object to provide a novel means "for automaticallycontrolling-"to a predetermined value the amount of radio frequencyenergy delivered by a waveguide.

In one aspect, the invention comprises a waveguide having a dielectricor insulating sheet disposed in the waveguide in the direction of theelectric vectors of the The dielectric or insulating sheet is covered onone side with photoconductive crystals or a photocond ive powder. Alighlmrcmgumlei s o riented with relation to an aperture in a sidewallof the waveguide so that light in varying amounts may be'directed ontothe photoconductive material. The photoconductive material has a veryhigh resistance in darkness, and the resistance decreases as a functionof the amount of light which falls on the photoconductive material. Witha maximum amount of light dmhotoconductive material, the resistance ofthe material is so low as to permit currents therein, and to therebyattenuate or reflect radio frequency energy passing thru the Waveguide.

Features of the invention include an arrangement whereby aphotoconductive material is mounted in a gap structure in the waveguide;a forked waveguide arrangement whereby the division of radio frequencyenergy in the two forks is readily controllable; and a constant poweroutput waveguide arrangement wherein the attenuation of energy in thewaveguide is automatically controlled to provide a constantpredetermined output.

These and other objects and aspects of the invention 1 will be apparentto those skilled in the art from the more detailed description taken inconjunction with the appended drawings wherein:

Figure 1 is a perspective view of a section of the waveguide includingan attenuator constructed according to the teachings of this invention;

Figure 2 is a cross section of a waveguide including a modified form ofattenuator;

Figure 3 is a view of a forked waveguide wherein each of the forksincludes a light-controlled waveguide attenuator for controlling thedivision of energy in the two forks; and

Figure 4 is an illustration of a waveguide including means toautomatically maintain a constant radio frequency output powertherefrom.

Figure 1 shows a light-controlled waveguide attenuator. The waveguide 10is the conventional rectangular metallic waveguide thru which radiofrequency electrical energy may be propagated with the electric fieldvector extending in the vertical direction between the greater sidewalls11 and 12. Waveguides of other shapes may, of course, be employedprovided that consideration is given to the direction of the electricfield vector in the waveguide. A sheet 13 of dielectric or insulatingmaterial is mounted in the waveguide to extend vertically between thegreater sidewalls 11 and 12 and to extend longitudinally thru thewaveguide for a limited distance. The insulating or dielectric sheet 13does not significantly interfere with or effect the propagation of radiofrequency energy thru the waveguide.

One side of the insulating sheet 13 is coated with a layer 14 ofphotoconductive crystals or a photoconductive powder. Thephotocondutcive particles may or may not be arranged so that theycontact each other and form a surface extending from the greatersidewall 11 to the greater sidewall 12. Any suitablejgonding techniquemay be employed to support the photoconductive particles. The bondingmaterial may be such as to provide a selfsupporting structure so that aninsulating sheet 13 is not needed.

The smaller sidewall 15 of the waveguide 10 is provided witlr anaperture 16. A light bulb 17, or other source of light, is disposedoutside the waveguide 10 and is oriented with relation to the aperture16 and the photoconductive layer 14 so that light from the source 17 maybe directed thru the aperture 16 to the layer 14. The amount of lightdirected from the source 17 to the layer 14 may be varied by varying theelectric current applied to the light source 17 or by placing acontrollable light barrier, for example an opaque shutter, betweenaperture 16 and source 17.

In the operation of the attenuator of Figure 1, radio frequency energyis directed or propagated along the waveguide 10 in the direction of thearrow 18. When the light source 17 is deenergized and no light'falls onthe photoconductive layer 14, the photoconductive layer has a very highresistance so that it is in effect an insulator. Under this condition,the radio frequency energy propagated thru the waveguide is unaffectedby the presence of the photoconductive layer 14 therein. When light fromthe source 17 is directed to the photoconductive layer 14, the layer 14becomes conductive as a function of the amount of light falling thereon.When the photoconductive layer 14 becomes conductive, the radiofrequency energyin the waveguide 10 is to some extent dissipated in theconductive layer 14 .and is therefore prevented from being propagatedundiminished thru the waveguide. It is apparent that the resistivity ofthe layer 14 is a function of the amount of light falling thereon, andthat the amount of radio frequency energy propagated thru the waveguidemay be controlled by controlling the amount of light from the source 17.Variations in the attenuation in the waveguide may be made as rapidly asthe light source 17 may be varied in intensity; subject only to thelimitations of the response time of the particular photoconductivematerial employed.

Figure 2 shows a modified form of the invention wherein the greatersidewalls 11' and 12' of the waveguide are provided with conductiveprotuberances 20, 21 to form a gap structure therebetween. A crystal 22of photoconductive material is mounted in the gap formed by theprotuberances and 21. Light is directed from a source (not shown) thruan aperture 16 to the crystal 22. The gap structure is such as toconcentrate or intensity the radio frequency electric field across thecrystal .22. Varying amounts of light falling on the crystal 22 producevarying degrees of attenuation of the radio frequency energy.

In Fig. 2, the photoconductive crystal 22 may be a single crystalmember, or may e a capsule of crystal particles held in plagehlgtransparent shell or by a suitable bonding agent. The gap structure andthe photoconductive matEi-ial therein may be elongated to extend anappreciable distance along the length of the waveguide. In Figure 2, ametallic screen 25 is placed over the aperture 16' to prevent the escapeof radio frequency energy from the waveguide thru the aperture 16'. Thescreen allows light to 'pass thru while confining the radio frequencyenergy in the waveguide. On the other hand, the aperture 16' may be madesufiiciently small so that radio frequency energy cannot escape eventhough the metallic screen 25 is omitted.

Figure 3 shows a waveguide 26 which is forked to provide two branches 27and 28. Each of the branches is provided with a layer ofphgtggonduntixc. material supported by an insulating sheet after themanner described in connection with Figure l. A light source 29 isassociated with one branch 27 and a light source is associated with theother branch 28. Radio frequency energy which passes thru the waveguide26 may be directed to either the branch 27 or the branch 28 by thesynchronous operation of the light sources 29 and 30. When light--source 29 is on and lightsource 30 is off, energy is directed thru thebranch 28. Conversely, when light source 30 is on and light source 29 isoff, energy is directed thru the branch 27.

Referring to Figure 4, a waveguide 32 is provided with a photoconductivelayer 33 which may be illuminated thru an aperture 34 from a lightsource 35. A radio frequency detector 36 has an input coupled by meansof a coupling loop 37 in the interior of the waveguide 32. The out- 0 toa put of the radio frequency detector 36 is applied power amplifier 38.e output of the power amplifier is applied over leadm to the lightsource 35.

In operation, radio frequency electrical energy isapplied to'"thewaveguide 32 at the left end thereof, from which it is propagated pastthe photoconductive layer 33,

and then past the coupling loop 37 to the output end 40 of thewaveguide. A constant value of radio frequency power may be deliveredfrom the output end 40 of the waveguide 32 while a varying amount ofpower is applied to the input end 41. When a greater than predeterminedvalue of radio frequency energy is coupled from the waveguide by thecoupling loop 37, the gligil'gqggltwk En land..app isgislls ifiwflwl -tuas The light frorn the source is is t hus increased to cause theresistance of the photoconductiveda'm to decrease. ;As a result, radiofrequency energy is attenu: ated until the desired predetermipedigltggpfsenergsnis,

delivered at the'ioutputendi til of the waveguide 32-. It 7 is apparentthat the system operates as an automatic power control to maintain aconstant power output regardless of variations in the input power to thewaveguide.

There are various photoconductive materials well known in the art whichare suitable for use in the attenu- 4 ators of this invention. is one ofthe well known phgtials w ich have been found to be particularlysuitable.

The term waveguide" as used herein is not limited to waveguides of thetype commonly used for the transmission of radio frequency energy fromone point to another. The term waveguide" is intended to includestructures of all kinds-thru which radio frequency energy is propagatedwith the electric field vector in a predetermined orientatron.

What is claimed is:

l. A light-controlled waveguide attenuator comprising a waveguide havingopposed walls through which radio frequency energy may be propagated,photoconductive material in said waveguide, a support for saidphotoconductive material positioned between said walls, one of said.

walls being provided with an aperture therein, said aperture being in aposition to permit light from a source to fall upon said photoconductivematerial so that said material serves to attenuate radio frequency.energy passing through said waveguide.

2. A light-controlled waveguide attenuator comprising a waveguidethrough which radio frequency energy may be propagated, said waveguidebeing rectangular in shape and having greater and lesser side walls, oneof said lesser side walls being provided with an aperture therein,photoconductive material positioned in said waveguide and extending fromone of said greater side walls to the other of said greater side walls,and a light source external of said waveguide, said aperture being in aposition to permit light from said source to fall upon said material sothat said material serves to attenuate radio frequency energy passingthrough said waveguide.

3. A waveguide attenuator as defined in claim 1, wherein saidphotoconductive material is in the form of photoconductive powder, andin addition a dielectric sheet in said waveguide supporting said powder.

4. A waveguide attenuator as defined in claim 1, wherein said lightsource is electrically operated, and in addition, a variable source ofelectricity connected to energize said light source.

5. A waveguide attenuator as defined in claim 1, wherein said lightsource is electrically energized, and in addition, a radio frequencydetector having an input coupled to the interior of said waveguide, anda power amplifier having an input coupled to the output of saiddetector, and having an output coupled to said light source, theintensity of said light source being responsive to said output of saiddetector, whereby the radio frequency energy passing thru said waveguideis automatically controlled.

6. A waveguide attenuator comprising, a waveguide having opposed walls,conductive protu'aerances extending from opposite. walls. of saidwaveguide to form a gap from which two waveguide branches extend, eachof said branches having a wall with an aperture therein. a sheet ofphotoconductive material in. each of said branches extending in thedirection of the electric field vectors therein, and means external ofsaid waveguide to direct light through said apertures to saidphotoconductive sheets in said branches, whereby the radio frequencyelectric energy passing thru said twobranches may be varied by varyingthe light directed thereto.

8. A light-controlled waveguide attenuator comprising a waveguide havingopposed walls through which radio frequency energy may be propagated, alight source external of said waveguide, one of said walls beingconstructed to permit light from said lource to be transmitted through aportion thereof, a sheet of photoconductive material positioned in saidwaveguide to receive light from said source, said photoconductivematerial being positioned so that its surface is substantially parallelto the I electrical field vector in said waveguide and so that when saidmaterial is illuminated by said light source it serves to attenuateradio frequency energy passing through said waveguide.

6 References Cited in the file of this patent UNITED STATES PATENTS

